1. Field of the Invention
The present invention relates to a sitting unit, such as a chair, and more particularly to a work station chair which can shift in response to a person leaning forward in the chair when in a working mode and leaning back in a rest position.
The present invention further relates to a backrest for a chair which pivots around an axis coincident with the H point axis relative to the chair. In addition, the present invention also relates to a chair which can shift in response to a person sitting on the chair, the chair also comprising a backrest which pivots about an axis coincident with the H point axis relative to the chair.
2. Description of the Prior Art
There have been many attempts in recent years to provide a work station chair which is ergonomic and which will adapt to the different positions assumed by a person working at a desk, table or computer.
A conventional rocking chair has certain advantages in this regard since it allows the person sitting on the chair to lean back in a rest position, wherein the person body can be relaxed and the back can be completely supported by the back rest. In a working position, the person normally leans forward over the desk or table, back erect. In a conventional non-rocking chair, the person would tend to sit on the front edge of the chair, while a rocking chair tilts forward with the shift in the center of gravity to continue to support the person's buttocks. However, a rocking chair is not practical. The center of gravity of the body must be relatively low in a rocking chair, thus preventing its use as a high chair as in the case of a drafting chair. Furthermore, it cannot be adapted with wheels. Most working station chairs must be equipped with wheels in order to allow the chairs to be easily displaced.
An improvement in terms of a working station chair but which must still be classified as a rocking chair is the chair described in U.S. Pat. No. 4,738,487, issued Apr. 19, 1988 to Shalinsky et al. In the Shalinsky et al patent, the chair is pivoted at the base such that the seat "rocks" forwardly and rearwardly to follow the person's center of gravity. The Shalinsky et al chair is also limited as to height since the arc of travel of the seat is determined by the radius from the pivot point on the base.
Another category of chairs for work stations which aims to allow passive forward and rearward movement of the seat in order to follow the attitude of the person leaning forward in a working mode or leaning back in a rest mode, includes the Setbar U.S. Pat. Nos. 4,650,249, issued Mar. 17, 1987, and the Cowan et al. U.S. Pat. No. 5,048,893, issued Sep. 17, 1991. In these patents, an arcuate track is provided at the top of the post and a carriage travels in the track with a seat mounted on the carriage. The arrangement allows the seat to passively adjust itself with the person's body in relation to changes in the center of gravity of the body.
However, it has been found that although the above chairs provide reasonable adjustment in the fore and aft directions and allow for tilting of the seat, they provide a compromise in terms of vertical adjustment. In fact, when a person leans forward to work, there is a vertical upward component to the movement. This movement is a somewhat rotational movement pivoting about the knees or ankles of the person. Likewise, when the person leans back in & rest position, there is a natural downward vertical component to the movement which is a rotation in the opposite direction pivoting about the ankles or knees.
Conventional backrests rotate around an axis located below the seat or at the back thereof. Upon tilting of such a backrest, the movement of the dorsal support member of the backrest is greater than that of the upper part of the body of the user in contact with the dorsal support member. The greater relative movement of this dorsal support member, as compared to that of the upper body of the user, is a disadvantage, since a discomfort is produced from that friction. For example, the relative movement Of the dorsal support member tends to pull out the shirt tail tucked inside the pants of the user leaning against the backrest.
It has been previously realized that when the backrest pivots about an axis which is essentially coincident with an axis passing through the hips (H point) of a person properly positioned on the chair, the movement of the backrest then coincides with the movement of the body, such that no such relative movement occurs between the dorsal support member and the upper part of the body of the user. Therefore, no friction occurs and no discomfort is felt. U.S. Pat. Nos. 5,024,484 and 5,052,753, both by Jurek Buchacz, describe a backrest which pivots about a point above the seat and in front of the backrest and including the H point. However, the mutual adjustment means permitting the sliding of the backrest in the seat in U.S. Pat. No. 5,052,753, as well as the frame members and runners described in U.S. Pat. No. 5,024,484, offer too much friction to permit a smooth operation of the backrest. Furthermore, the backrests taught in these two U.S. Patents are mounted on conventional seats or on seats which also pivot about the H point axis.
The H point, actually an H axis, is defined as the pivot center of the torso and thigh (Society of Automotive Engineers (SAE) Handbook, 1986, p. 34-55). As seen in FIG. 1 of the SAE Handbook (p. 34-33), the H point is defined as the point of intersection of the torso line and the hip axis. It is important to note that, while anatomical characteristics such as lower leg segment or thigh segment can vary significantly according to the user, the H point remains relatively constant. Thus, by taking into account parameters such as the cushion compression of the seat upon proper sitting of the user, and the buttocks segment of the majority of users (for example, the 95th percentile), the definition and location of the H point of the majority of users can be accurately predicted. From this H point, an H point relative to the chair can be positively located on the chair.